This invention pertains to a cathode for a tantalum electrolytic capacitor, and more particularly to a metal composite cathode having an inner layer of tantalum, a copper middle layer and a nickel outer layer. A layer of tantalum carbide formed on the tantalum interior layer of the cathode is covered by a layer of finely-divided carbon.
The cathode for wet tantalum electrolytic capacitors should be able to withstand reverse polarity voltage. When a silver cathode is used, such reverse polarity causes dissolution of the silver with silver ions going into solution in the electrolyte. Under prolonged operating conditions, the silver tends to come out of solution in the form of needles which can bridge the space between cathode and anode to short out the capacitor.
To remedy this silver migration problem, gold, silver-gold alloy, or gold-platinum alloy layers have been applied to a less-expensive base, e.g. copper, and used in the prior art as the cathode. Alternately, expensive all-tantalum cans have been used in the prior art, but an air oxide film which readily forms on tantalum prevents good electrical contact between a tantalum cathode and the electrolyte. There has also been a problem in adhesion between tantalum and materials like platinum black. For these reasons, a tantalum carbide layer which does provide good electrical contact has been used on tantalum cans.